Mobile radio communications originally involved the communications directly from one mobile radio to a second mobile radio. It was quickly realized that the range of communications was limited by virtue of the low antenna heights and antenna gains associated with mobile radios. A repeater was devised as a radio transmitter and receiver which could be mounted with an antenna at a higher elevation thereby increasing the range of mobile communications. Because of the need for the repeater to both transmit and receive, a scheme for allocating frequencies was devised whereby a channel comprised two frequencies, a transmit frequency and a receive frequency. As such, a mobile radio originating a call could transmit on the first frequency which would be received by the repeater receiver. The repeater coupled the receive signal to its transmitter operating on the second frequency and the transmitter broadcasts the signal over a large coverage area where it could be received by a second mobile radio receiving on the second frequency. The concept of coupling the repeater's receiver to the repeater's transmitter is called repeat audio.
Since most users do not communicate continuously while using land mobile radio services it was realized that a single repeater could service a number of groups of users. Users who wished to communicate using the land mobile radio system would have to wait until the repeater was free and available for communications before initiating communications.
Originally, radios controlled their receive audio by using the carrier squelch scheme. With the carrier squelch scheme the radio mutes the received audio until the radio frequency carrier is detected. In this scheme, all users and all groups could listen to the communications of other groups because each of their radios would open squelch every time the radio frequency carrier was detected.
A later improvement involved the use of a coded squelch scheme. Such a scheme employs tone sequences which are detected by the receiving mobiles to cause them to open squelch only upon the detection of a tone sequence which had been preprogrammed into the receiver. In this way multiple groups of users could share a common repeater yet only listen to conversations intended specifically for them. Each transmitting mobile would transmit the tone sequence coding the squelch signal and the receiving mobiles would only open their receiver speakers in the presence of the same coded tone sequence.
A still later improvement was the use of out of band signalling to control squelch. Such a signal may use subaudible tones or subaudible data streams to control the squelch in radios. Motorola Corporation developed an early system called Digital Private Line that utilized a subaudible data stream to control the squelch in receiving mobiles.
As the number of users increased, the loading, or the total quantity of users in groups accessing the given repeater increased dramatically. With the heavy loading, the grade of service begins to degrade as users who requested service are unable to achieve it because other users were currently utilizing the given repeater.
Yet another improvement in the use of land mobile radio involved the trunking of a plurality of repeaters together to enhance the grade of service to users. For such a system to function, the radios needed to be frequency agile. The use of synthesized transceivers provided for this capability. In such an environment, it is necessary not only to control the squelch in the radios but also the channel management, that is, the access of frequencies. Several different system protocols were developed. Motorola developed a system whereby a dedicated control channel was used within a trunking group of channels. The dedicated control channel was utilized by mobiles to request a voice channel and the control channel in turn acknowledged the use of a particular voice channel by the mobile.
The E.F. Johnson Company of Wauseka, Minn., developed a system called Logic Trunk Radio. The acronym for that is LTR.RTM.. In the LTR.RTM. system, each repeater transmits a subaudible data stream that is used for channel management and squelch control by the mobiles. The subaudible data is transmitted in data frames which are transmitted repetitively by the repeaters. When a repeater is idle, each repeater transmits a data frame approximately every ten seconds to indicate to the fleets of mobiles that the repeater is functioning and within range. Because the LTR.RTM. system was a open system, it became a defacto standard for a portion of the land mobile radio industry. Many manufacturers made compatible radio equipment and also compatible repeater and repeater logic equipment.
The LTR.RTM. system was designed around a single trunking group of repeaters which would naturally operate from a single radio transmitter site. Multiple systems could be installed in different geographical areas, however, there was no coordination between the multiple systems.
The LTR.RTM. system provided basic dispatch service in a transmission trunk mode of operation. As each radio user keys their microphone to initiate a transmission, the radio handshakes with the repeater using the subaudible data protocol and opens the repeat audio in the repeater which rebroadcasts the message being transmitted by the mobile radio. As soon as the user dekeys his microphone, the repeater turns off the repeat audio and disables its transmitter. Subsequently, as a message is responded to by a different mobile within a group of mobiles, the repeaters are rekeyed, the repeat audio turned on and a reply to the message is broadcast back. It is important to understand that each transmission is a relatively short burst lasting just a few seconds, and, for each transmission the system may trunk the user to a different available channel. The subaudible data stream, handling the channel management, would assign all of the mobiles within the group intended to receive the call to the appropriate repeater at the beginning of each transmission.
An alternative form of communications would be to interconnect the repeater to the public switch telephone network and allow the radios to access the telephone network for conversations. In this mode of operation, commonly called conversation trunked mode, the repeater is kept in the conversation for the duration of the call. Unlike transmission trunked operation, in conversation trunked operation, the length of calls is relatively long. Also, it is necessary to assign a unique ID for use in telephone interconnected service.
The nature of trunking systems which is well understood by those skilled in the art it is known that short bursts of communications utilize a trunked group of resources more efficiently than long duration communications.
Considering again that an environment where there are multiple radio transmitter sites, each having a plurality of repeaters in a trunked group, users who had access to such an environment had a need to communicate not only with one site but across the plurality of sites. It is understood in the industry that people with the capability to communicate continually wish to expand the area over which they communicate and increase the grade of service and the features available to them. The LTR.RTM. system did not provide a structure either in ID codes or in communication switching that facilitated a multiple site integrated environment. Furthermore, the LTR.RTM. system had a finite set of ID codes that could be used for either group conversations or for individual interconnected types of conversations. In a large network with a plurality of sites, where users must access multiple IDs at multiple sites, the resource and availability of ID codes was quickly diminished and the grade of service reduced. The reduced grade of service in conjunction with the inability to manage intersite communications left the LTR.RTM. system incapable of providing the needs of the end users. There is a need for a land mobile radio system wherein a plurality of sites each having a trunking group of repeaters can be interconnected and switch in such a way as to provide uniform integrated service across the entire network coverage area. Furthermore, an ID code structure is needed that allows for extended ID code naming sequences and also naming sequences for individual trunked repeater groups.
Also, there is a need to integrate not just land mobile radios with the public switch telephone system, but the need to use other network linking resources which can also be easily integrated into the network. With the operation of a complex network system, it is necessary to have management tools which can be used for configuring the network, monitoring the operation of the network and maintaining the network.